hardest Zelda? Maybe, can't really tell. For me, most of the difficulty came from struggling with the controls
fixed rupees? How? The items upgrades? Well, it's a step in the right direction
overworld? SS has no overworld
SS definitely fixed rupees. Unless for some reason you don't buy anything, you regularly have to use the rupees you find in a way that makes it rare for you to have a full wallet. You need to buy potions (fairies are much less common) and gear, upgrades for said gear and wallets and other stuff in Beedle's shop. In short, rupees are useful for something and it should be rare to find rupees with your wallet full.